1. Field of the Invention
The invention relates to a method for operating a wireless mesh data network comprising a plurality of network nodes, in which data frames are transferred from a source node via one or more intermediate nodes to a destination node, wherein the source node, the one or more intermediate nodes and the destination node constitute network nodes of the data network.
2. Description of the Related Art
The transfer of data frames between the source node and the destination node can in principle occur over different routes in mesh data networks. A route is understood to be a number of network nodes which are arranged adjacently relative to each other and which, at their ends, have a data connection to the source node and the destination node. In order that the transfer of the data frames from the source node to the destination node is not left to chance, a so-called route request is broadcast from the source node to all adjacent network nodes, which likewise in the context of a broadcast forward the route request to their adjacent network nodes, until the route request finally reaches the destination node. A so-called route reply is initiated by the destination node. During the transfer of the route request, and during the targeted return transfer of the route reply (i.e., unicast) to the source node, entries are created in so-called routing tables on each network node. This results in a defined path for the transfer of data frames between the source node and the destination node.
In the context of the present invention, therefore, a path or data path (or route) is understood to represent the transfer of data frames between the source node and the destination node through one or more specified intermediate nodes. Data frames which are transferred from the source node to the destination node along the data path are transferred on a so-called forward route. When data frames are transferred from the destination node to the source node, this is referred to as a so-called reverse route in the following description.
In the case of such wireless mesh data networks, the problem exists that individual data frames can be misdirected on a path between the source node and the destination node, thereby causing loops or data loops. This adversely affects the data communication between the source node and the destination node. The incorrect transfer of data frames can occur in all known transfer protocols (i.e., routing protocols). The formation of such unwanted data loops can occur as a result of faulty network nodes, by chance or by malice when data frames are transferred to a network node which precedes the transferring network node on the path in the transfer direction. As a result of the loop formation, the data frame or data frames are forwarded to a network node which is located closer to the source node in a forward direction on the path. As a result, the available bandwidth of the network can be substantially reduced.
In order to prevent the continuous transport of a misdirected data frame, it is customary to integrate lifetime information in the data frames. This is done in the case of data packets according to the Internet protocol (IP) and in the case of data packets as per the IEEE 802.11s specification, for example. The lifetime information is referred to as Time-to-Live (ttl). This is an integer value, which is normally set to 255 by the sender of the data frame. Each time the data frame is forwarded to a new network node, the value is decremented by 1. As soon as the lifetime value is 0, the data frame is discarded and is no longer transported onward in the data network.
A further known protection mechanism is referred to as “source routing”. In this context, the recipient of a data frame can check whether the sender (transmitter) of the data frame is authorized to forward this data frame to the recipient. Here the, check is based on path information contained in the data frames.
It is also known to use unique sequence numbers when transferring data frames. This makes it possible to verify whether data frames have already been transferred. With reference to the sequence numbers, it is then possible to infer a loop formation if a data frame arrives a second time or repeatedly at a network node.
The use of so-called precursor lists is known from the Ad-hoc On-demand Distance Vector Routing (AODV) algorithm as described in RFC 3561 for IP MANET routing. The precursor lists are used for error notifications of the network nodes. Here, a precursor list is generated both for the forward direction and for the reverse direction of a data path. This occurs during the processing of the route reply which was broadcast by the destination node. This procedure is based on both directions (forward direction and reverse direction) following the same path. However, the precursor lists used in the context of AODV cannot prevent the occurrence of loops in this way.